Multiple myeloma (MM) is the most frequent cancer to involve the skeleton and induces osteolytic lesions that rarely heal in both axial and craniofacial bones. Multiple myeloma bone disease (MMBD) is responsible for some of the most devastating complications of MM and is the major source of morbidity associated with MM. Bone marrow stromal cells (BMSC) are a major type of cells that reside within the MM microenvironment. It has been shown that in MMBD, BMSC produce many growth factors and inflammatory cytokines. These factors can boost the growth of the myeloma tumor cells and activate osteoclasts, the bone resorbing cells, to induce osteolytic lesions in bone. Thus, disrupting the BMSC support of MM cell growth and osteoclast formation is of major clinical significance in treating MMBD. Our long-term goal is to elucidate the molecular mechanisms that regulate BMSC support of MM cell growth and bone destruction in MMBD and identify the potential therapeutic targets for disrupting BMSC support of MMBD. Towards this goal, we have found that a cellular stress molecule spliced X-box-binding protein 1 (XBP1s) is induced in the BMSC derived from MM patients, compared with those from the healthy donors. XBP1s has been shown to control gene expression and/or protein secretion of inflammatory cytokines in other organs and disease models, such as inflammatory bowel disease. We showed recently that elevation of XBP1s protein levels in healthy donor BMSC induced the pathological behavior that are usually present in MM patient BMSCs, such as, heightened inflammatory cytokine secretion, enhanced support of MM cell growth and OCL formation both in vitro and in vivo. Conversely, knockdown of XBP1s in MM patient BMSC largely corrected their pathological behavior to the levels that are comparable to healthy donor BMSC. In this RO1 grant application, we hypothesize that IRE1?/XBP1s signaling is an essential pathophysiological factor that regulates the BMSC inflammatory signature and BMSC support of MM cell growth and osteoclastogenesis. Thus, the IRE1/XBP1s signaling in BMSC is a potential therapeutic target for disrupting BMSC support of MM cell growth and bone destruction in treating MMBD. The Specific Aims are: Aim 1: To determine the pathophysiological significance of p38-induced phosphorylation of human XBP1s (hXBP1s) in BMSC support of MM cell growth and osteoclastogenesis both in vitro and in vivo. Aim 2: To determine whether RANKL is a novel transcriptional target of XBP1s. Aim 3: To determine whether deletion of Xbp1 in BMSC blunts MM cell growth and bone resorption in vivo using a novel immunocompetent BMSC-specific Xbp1 KO mouse model. Aim 4: To determine if the IRE1? endoribonuclease activity in BMSC represents a potential therapeutic target to repress generation of XBP1s and disrupt BMSC support of MM cell growth and OCL formation. These studies have multiple biological, pathological and clinical implications. First, our studies will provide important information and related animal models for developing and employing therapeutic strategies that target the IRE1?/XBP1s signaling, such as the existing IRE1? inhibitors, and/or inflammation kinases-induced phosphorylation of XBP1s to disrupt the protective effects of the MM microenvironment on MM cells and OCL as a means to treat MMBD. Secondly, these studies will not only advance our understanding of basic biology of XBP1s but also provide important information on potential impact of an IRE1?/XBP1s inhibitor on bone microenvironment homeostasis of MM patients. Thirdly, since heightened stromal inflammatory cytokine secretion is a common pathological feature of many inflammatory bone diseases, such as rheumatoid osteoarthritis and tumor bone metastases (e.g., prostate, breast and lung cancers), our studies will provide important information and related animal models to investigate if the IRE1?/XBP1s signaling in BMSC is also a critical pathological factor in regulating the stromal cells support of progress of these inflammatory bone diseases, and thus represents a potential therapeutic targets for treating these inflammatory bone diseases.